GaN, AlN, and InN, etc., which are nitride semiconductors, or materials made of mixed crystals of these nitride semiconductors, have a wide band gap, and are used as high output electronic devices or short-wavelength light emitting devices. Among these, as high output devices, technologies are developed in relation to field effect transistors (FET), more particularly, High Electron Mobility Transistors (HEMT) (see, for example, Patent Document 1). A HEMT using such a nitride semiconductor is used for high output/high efficiency amplifiers and high power switching devices.
As a field effect transistor using a nitride semiconductor, there is a HEMT in which GaN is used as an electron transit layer and AlGaN is used as an electron supply layer. In the electron transit layer, 2DEG (Two-Dimensional Electron Gas) is generated, by the functions of piezoelectric polarization and spontaneous polarization in GaN. As this kind of HEMT, there is a HEMT having a MIS structure, in which an insulating film is formed on the electron transit layer and a gate electrode is formed on the insulating film. In this way, by forming an insulating film between the gate electrode and the electron supply layer, a gate leakage current can be suppressed.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-359256
Patent Document 2: Japanese Laid-Open Patent Publication No. 2012-174804
Patent Document 3: Japanese Laid-Open Patent Publication No. 2010-50347
Patent Document 4: Japanese Laid-Open Patent Publication No. 2010-118556